As is well known, processes for preparation of single crystal thin film, polycrystalline thin film and the like are of two types: dry processes and wet processes. A dry process is more often used, however, since the thin film prepared by a dry process is generally superior from the aspect of quality to that by a wet process.
Dry processes include physical film preparation techniques such as the vacuum deposition method, the ion plating method and the sputtering method and chemical film preparation techniques such as the chemical vapor deposition method (CVD method). Among these, the latter CVD method is widely employed because it is suitable for mass production owing not only to the fact that deposition rate control is easy but also to the fact that film preparation need not be conducted under high vacuum and, moreover, that high-speed film preparation is possible.
In the CVD method, a metalorganic complex is used as the precursor compound. In the case of decomposing the vapor thereof to form a metal thin film or the like, the thermo-CVD method, photo-CVD method or plasma-CVD method is adopted. Particularly in the cases of preparing the ferroelectric materials important in the development of recent memories such as the DRAM and FRAM and preparing the oxide thin films of the sub-electrode etc. constituting the underlayers thereof, the complex used as the precursor has generally been a metalorganic complex whose organic portion (ligand) is dipivaloylmethane, a kind of .beta.-diketone. This is a dipivaloylmethane (DPM) chelate of M divalent metal represented as M(DPM).sub.2, where M is a divalent metal.
Since the melting point of such a metalorganic complex is generally high, however, the precursor vapor has to be generated by sublimation from the solid. In the case of a metalorganic complex whose melting point is higher than 200.degree. C., for example, the operating devices of the exhaust system (automatic vacuum valves in the tubing, for example) must be made of a material that can withstand temperatures above 200.degree. C. Fabrication of a mass production apparatus of this type is difficult with current technology. In view of equipment requirements, therefore, vaporization from solid state below the melting point is unavoidable.
In vaporization from a solid precursor, however, since it is difficult to generate gas of saturated vapor pressure for film preparation, a problem arises that the amount of precursor vapor obtained varies with change in precursor surface area. In other words, the surface area of a continuously used solid precursor decreases because of diminishing quantity and condensation of the precursor in the source vessel, resulting in a gradual lowering of the vaporization rate. This is particularly impractical in multi-element system thin film preparation because the decrease in vaporization rate causes variation in composition.
A need has therefore been felt for development of precursor compounds that can be vaporized from liquid state. This is because a liquid precursor readily provides a precursor gas of saturated vapor pressure when bubbled and does not give rise to difference in deposition rate owing to time-course change in the amount of precursor in the vessel.
By use of some .beta.-diketones other than DPM as ligand, we earlier developed CVD precursor compounds (metalorganic complexes) containing Cu, Pb, Y, Nd, Ru, Ir etc. that have much lower melting points than DPM complexes and can be used in liquid state. However, with regard to the alkaline earth metals, which are important constituent elements of ferroelectric thin films, oxide superconducting thin films and so forth, virtually no metalorganic complexes with characteristics superior to the corresponding DPM complexes are known.
On the other hand, when ferroelectric materials such as (Ba, Sr)TiO.sub.3 (BST) and SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT) are prepared by the CVD method, securement of a uniform high deposition rate by high quality vaporization of the Ba, Sr or other precursor compound is indispensable. A liquid source CVD method has therefore recently been developed in which, even when using an alkaline earth DPM complex as the precursor compound, the precursor compound (the DPM complex) is once dissolved in a solvent, the solution is fed to the vaporizer under flow control by a fluid mass flow controller, and the whole amount, including the solvent, is instantaneously vaporized.